In recent years, a carbon brush for an electric machine intended for a commutator motor (hereinafter it is simply referred to as “the brush”) has progressed on to miniaturization, high-power and high-velocity revolution. Accordingly, the brush of compact, less wearing, and less-temperature-rise under the high electric current density has been increasingly required.
However, the existing brushes have the tendency that under the high electric current density and high-velocity revolution, their commutating properties deteriorate to produce increase in wearing and temperature rise of the brush. Accordingly, the miniaturization of the brush is not advanced so remarkably in the present situation as the miniaturization of the commutator.
As it is generally known, when the brush of high resistivity is used for the commutator motor, reliable rectification is obtained. This is because the brush of high resistivity prevents short-circuit current from flowing between adjacent commutators by way of the brush. On the other hand, when material of high resistivity is used, the brush itself resistance heating and produces an increased temperature due to the high resistivity. Further, as the motor is increasingly advanced on to high-power, miniaturization and high-velocity revolution, the current flowing through the commutator increases and thus the temperature of the commutator increases. Due to this, an excessive coating of the commutator could produce stick-slip and in turn produces an increased commutation spark, then causing a further increase in temperature and wearing of the brush.
In the electric machinery of high revolution speed such as an electric vacuum cleaner, a resin-bonded brush comprising graphite powder bonded by a binder is sometimes used as demanded, in order to provide improved rectification and provide a brush free from replacement during the use of the electric vacuum cleaner body. However, as a result of a long hours use of the cleaner, the temperature rise is produced, then creating a vicious circle that the lubricating property of the brush itself is deteriorated to produce further temperature rise.
In Japanese Laid-open Patent Publication No. Hei 5(1993)-182733, the inventors disclosed the technical improvement that a metal having good electrical conducting properties, such as nickel, copper, gold and silver, is coated over outer surfaces of the brush material, whereby an apparent resistance is decreased to thereby suppress the temperature rise. Although this technical improvement of Japanese Laid-open Patent Publication No. Hei 5(1993)-182733 can suppress the temperature rise to some extent, it cannot be said to be sufficient for the temperature rise resulting from the high power and high-velocity revolution in recent years.
On the other hand, Japanese Laid-open Patent Publication No. Hei 2-51345 discloses a method of producing the brush, according to which for the purpose of keeping lubricating properties of the brush under temperature as high as the brush temperature reaches, a solid lubricant agent, such as molybdenum disulfide or tungsten disulfide, and an abrasive are granulated and then added to a thermosetting resin and the mixture is applied to the brush material to thereby produce the brush. However, this method cannot be said to be sufficient for the temperature rise resulting from the high power and high-velocity revolution in recent years, either.
Accordingly, it is an object of the present invention to provide a carbon brush for electric machinery of less temperature rise and excellent wear resistance to meet the demand of high power and high-velocity revolution.